Many of the currently-available high performance planetary transmissions employ what is termed clutch-to-clutch shifting. This term indicates that the ratio change is performed by disengaging one disc-type friction device while engaging another disc-type friction device. This is accomplished without the use of one-way devices. Therefore, the overlap control needs to be quite accurate in these situations and the position of the control must also be accurate.
At least one planetary transmission that is utilized with clutch-to-clutch shifting controls is shown in U.S. Pat. No. 4,070,927 issued to Polak on Jan. 31, 1978. This transmission has a control that is equipped with solenoid controlled trim valves that are actuated by electronic control units to provide engagement and disengagement pressures for the torque-transmitting friction devices within the transmission. One such solenoid control is shown in U.S. Pat. No. 5,601,506 issued to Long et al. on Feb. 11, 1997. Also, the transmissions in this category use what is known as skip shifting, that is, a first-to-third ratio interchange or a second-to-fourth ratio interchange. The above-identified Long et al. patent does not provide for skip shifting.
It is also desirable to ensure that the vehicle incorporating these transmissions can be returned to a repair facility in the event of a discontinuance of electrical power, which is known as limp home capability. Such control systems can be found in U.S. Pat. No. 4,827,806 issued to Long et al. on May 9, 1989, and U.S. Pat. No. 5,616,093 also issued to Long et al. on Apr. 1, 1997.
The transmission controls utilize trim valves, which are operating in combination with shift valves to control the on-coming and off-going friction devices. The trim valves are equipped with variable pressure solenoids while the shift valves are controlled by on/off-type solenoid valves.
U.S. Pat. No. 6,520,881 issued to Long et al. on Feb. 18, 2003, describes a control system wherein four solenoid valves control four trim valves, which in turn control the on-coming and off-going pressures in five torque-transmitting mechanisms. This control mechanism incorporates two latching valves, which are multi-plexed to control fluid pressure distribution to three torque-transmitting mechanisms. Limp home capability is provided by the control system disclosed in this Patent. The control of U.S. Pat. No. 6,520,881 will permit limp home capability in either the third or fifth forward ranges